PSUN346 TSHoma: A Rare Cause of Pituitary Adenoma

INTRODUCTION: The TSH-producing adenoma (TSHoma) prevalence in the general population is 1 to 2 cases per million, accounting for less than 2% of all pituitary adenomas. Plurihormonal tumors account for about 30% of these cases, with the most common co-secreted hormone being GH. We present the case of a patient diagnosed with TSH adenoma many years after presentation. CASE PRESENTATION: A 58 year old male with a history of incidentally diagnosed pituitary microadenoma on MRI in 2007, presented in 2014 following abnormal thyroid function testing (TFT), only reported symptoms being occasional headaches. TFT was significant for elevated TSH and FT4 (TSH 4.87—Ref range: 0.27-4.2. Free T4: 1.98–Ref range 0.7-1.7). Pituitary hormone profile was remarkable for a low IGF-1 and a borderline high Alpha subunit of glycoprotein hormone. A sellar MRI confirmed a 12mm pituitary macroadenoma. He was managed conservatively on beta blockers and low dose Methimazole, owing to a stable adenoma size and poor surgical candidacy with multiple comorbidities (e. g. Atrial fibrillation). Repeat labs in 2018 showed increasing TSH (12.85), with persistent elevations of FT3, FT4 and Thyroglobulin. He had follow up TFT monitoring and surveillance MRIs until 2021 when his lesion progressed to 14 mm with mass effect on right pre chiasmatic optic nerve and adjacent internal carotid artery. The patient underwent trans sphenoidal biopsy with debulking in 2021. The biopsy showed a plurihormonal adenoma (prolactin, GH and TSH secreting). Interestingly, prolactin, GH or IGF-1 were never elevated. Following surgery, MRI showed residual 13.2×9.7 mm tumor with no mass effect. His most recent labs showed a TSH: 2.5 and FT4: 1.7. DISCUSSION: The diagnosis of a TSHoma should be considered in all centrally hyperthyroid patients, to avoid complications and undue thyroid ablation. A TSHoma must be differentiated from Resistance to Thyroid Hormone (RTH)-beta, which occurs due to mutations in the Thyroid Hormone Receptor Beta gene. Serum a-subunit concentration and Sex Hormone Binding Globulin (SHBG) concentrations are normal in RTH-beta with no TSH suppression following somatostatin analog administration. The presence of a high TSH, FT3 and FT4, in the setting of an elevated a-subunit and pituitary macroadenoma on MRI is strongly suggestive of a TSHoma. Beta blockers are recommended for symptomatic individuals however anti thyroid drug therapy is not advisable for there is risk of thyroid hormone suppression with concomitant tumor overgrowth. Definitive therapy includes trans sphenoidal tumor resection with somatostatin analog therapy to achieve euthyroidism prior to surgery. Patients should be followed up with frequent monitoring of TFT's, along with MRIs every 2-3 years. For patients with residual disease following surgery, treatment with somatostatin analogs such as Octreotide or Lanreotide is favorable as opposed to pituitary irradiation. Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.